Certain undesirable materials are found to be contaminants in wastewater. Water steams can be contaminated with substances like iron, manganese, organic matter, hydrogen sulfide, or bacteria. Iron causes taste and odor problems in potable water, causes staining in laundry, wash, swimming pool, or process water, and it causes fouling and deposits in boiler and cooling water systems. In many aqueous systems such as drain water, bilge water, grease traps, and holding tanks, odors can be caused by sulfides, mercaptans, and organic matter. These odors can be treated by oxidizing agents, but the oxidizers can be difficult to administer in low-flow or unattended areas. There remains a need for improved methods to treat metals, organics, bacteria, and odor compounds in water streams.
Wastewater management is a major problem in the petroleum industry. Petroleum industry wastewater includes oilfield produced water and aqueous refinery effluents. Petroleum industry wastewater also includes water used for hydraulic fracturing of oil-containing or natural-gas-containing geological formations.
Contaminants found in oilfield produced water and aqueous refinery effluents can include, at varying levels, materials such as: (1) dispersed oil and grease, if not removed by mechanical pretreatment separators can clog post-treatment equipment; (2) benzene, toluene, ethylbenzene and xylenes (BTEX), a volatile fraction that is usually handled by onsite wastewater treatments (WWT); (3) water-soluble organics, again usually handled by the WWT system; (4) sparingly soluble nonvolatile organics, including aromatics with molecular weights higher than BTEX but lower than asphaltenes, typically not removable by WWT systems; (5) treatment chemicals, such as drilling, completion, stimulation and production chemicals; (6) produced solids, usually removed by mechanical separators; and (7) total dissolved solids including metals, a particular problem because many metals are considered toxic. A variety of treatments are available to remove these contaminants, including the use of organophilic clays, carbon types, ion exchange resins, coalescers, coagulants, filters, absorbers, alpha hydroxy acids, dithiocarbamates for metals, and media filtration. There remains a need in the art, however, to identify more effective, efficient and cost-conscious solutions to these wastewater problems.
The urgency for improved wastewater management in the petroleum industry is heightened by rising public concern over environmental hazards and toxicities. For selenium, as an example, the U.S. Environmental Protection agency (EPA) plans to incorporate new discharge limits as low as 5 ppb. Current technologies for selenium removal include adsorption & precipitation, ion exchange, chemical or biological reduction, oxidation, and membrane treatment (nano-filtration or reverse osmosis). Even using these methods, it would be difficult and costly to meet the standards that the EPA is considering. Zinc and its compounds are another set of regulated inorganic contaminants in petroleum refinery wastewater. These compounds originate from many sources within a refinery including artificial addition, and require end-of-pipe treatment. Zinc compounds and other metals can be removed from wastewater using technologies such as lime precipitation, coagulation & flocculation, activated carbon adsorption, membrane process, ion exchange, electrochemical process, biological treatment, and chemical reaction to achieve in practical large scale. Some regulatory agencies have set discharge limits for these and other metals that exceed the capacity for commercial metals removal processes. A pressing need exists to improve methods for removing metals from wastewater in light of the increasing regulatory scrutiny of such wastewater contaminants.
Petroleum industry wastewater also includes water used for hydraulic fracturing. In the recovery of oil and gas from geological formations, hydraulic fracturing is a process of pumping fluids into a wellbore at high pressures to fracture the hydrocarbon-bearing rock structures. This fracturing increases the porosity or permeability of the formation and can increase the flow of oil and gas to the wellbore, resulting in improved recovery.
Hydraulic fracturing for hydrocarbon-containing formations typically uses water obtained from two sources: 1) surface water derived from water wells, streams, lakes, and the like, that has not been previously used in the fracturing process; and 2) water that has been used in, and/or flows back from fracturing operations (“frac flowback water”). Processes exist for treating both surface and flowback water sources to prepare them for use or re-use in hydraulic fracturing. Without appropriate treatment, contaminants entering the frac water can cause formation damage, plugging, lost production and increased demand for further chemical additives.
Iron in hydraulic fracturing water can cause corrosion, plugging of downhole formations and equipment, an elevated demand for frac additive chemicals, and membrane fouling in treatment processes. Techniques available for removing iron from frac water include aeration and sedimentation, softening with lime soda ash, and ion exchange. Aeration and other chemical oxidation practices are known for household well water treatment to remove iron. Oxidation converts the soluble iron (Fe+2) form to the less soluble iron (Fe+3) oxidation state, causing it to precipitate, often as iron hydroxide, which is collected by filtration or sedimentation. Greensand iron removal is one of the typical methods. However, greensand impregnated with potassium permanganate is only capable of treating iron concentrations up to a few ppm, while the iron concentration in oilfield frac flowback water and produced water can be as high as 300 ppm. Current methods of oxidant encapsulation and controlled release for soil and ground water remediation are not suitable for oilfield frac flow back water iron removal since the oxidant release rate is too slow for continuous flow through process. Ion Exchange and chelating resins cannot remove iron effectively from frac flow back water due to the co-existence of the high concentrations of other multivalent cations.
There remains a need in the art, therefore, to provide water treatment systems and methods that can remove iron contaminants effectively from water to be used in hydraulic fracturing, especially frac flowback water, where iron contaminants reach high levels. In addition, there remains a need for integrated water treatment systems that interface with the hydraulic fracturing processes efficiently, and that prepare water in a cost-effective way for use in these processes.